1. Field of the Invention
This invention relates to a projection lens unit and a video projection system for projecting and magnifying a picture of a relatively small cathode ray tube, liquid crystal display device, etc. onto a larger screen.
2. Description of the Prior Art
In order to obtain large size images, video projection systems are well known and widely used in various applications; such systems take a picture of a TV program or computer output from a relatively small cathode ray tube (CRT) screen and project and magnify it through a projection lens onto a larger screen. In most of these systems, the projection lens and the CRT are optically coupled by filling the gap between them with a highly transparent liquid in order to improve the clearness and contrast of the projected image. Such method is usually called Optical Coupling (OC).
One example of the prior art is shown in FIG. 7. Numeral 1 denotes a projection lens, and 2 denotes an OC housing having a CRT 3, with a screen on which a picture is produced, mounted on the right side and an OC lens 4, the rightmost component of the projection lens 1, mounted on the left side. The OC housing 2 is provided with a gutter 5, in which a sealing rubber 6 is affixed. When the OC lens 4 is mounted on the OC housing 2 from the left, the sealing rubber 6 is pressed and deformed to tightly stick to the surface of OC lens 4, and a watertight sealing is prepared. From the right side of OC housing 2, the CRT 3 is inserted up to a predetermined position, and in the gap between the inner surface of OC housing 2 and the side wall of CRT 3, a rubber string 7 having an even thickness slightly larger than the gap is pushed into covering the entire circumference of CRT 3 for the purpose of setting the up/down and right/left positioning of CRT 3 as well as preventing adhesive from escaping. Later on, the gap is filled with silicone rubber adhesive 8 for the watertight sealing.
At the top of the OC housing 2 is a pressure adjusting hole 9. A diaphragm 10, made of elastic material such as rubber, is mounted over the pressure adjusting hole 9. The diaphragm 10 has around its rim a pressure sealing part 11, which is seated in a sealing groove formed around the pressure adjusting hole 9. A holding cover 12 is mounted over the pressure adjusting hole 9 and diaphragm 10. The holding cover 12 presses the pressure sealing part 11 against the OC housing 2, thereby providing a watertight OC chamber 14 surrounded with OC housing 2, CRT 3, and OC lens 14. The holding cover 12 includes a ventilation hole 13 which exhausts air when the diaphragm 10 moves upward.
After the unit has been assembled as illustrated in FIG. 7, the OC chamber 14 is filled with an optical coupling liquid 15 (OC liquid), and the pressure adjusting hole 9 is covered with the diaphragm 10 and sealed watertight. Suitable OC liquids include ethylene glycol, ethylene glycol mixed with glycerin, and ethylene glycol mixed with glycerin and a small amount of pure water.
FIG. 8 outlines the structure of a widely used 3-CRT video projector. One projector employs three sets of projection opticals 21, 22 and 23 as described above; monochrome images each in red, green and blue are projected through each of the projection opticals onto a screen 60 to be composed there as a color image. The optical axis of projection opticals 21 in the middle is perpendicular to the screen, however, the optical axes of projection opticals 22 and 23 located in the sides each have an angle .theta. (this angle is called the concentration angle) to the screen. Namely, they are projected optically oblique.
FIG. 9 illustrates a projector (side view) hanging from a ceiling for projection on a screen 60 on a wall, etc. In this case, all the axes of the projection opticals 21, 22 and 23 are oblique to the screen 60. The axes of the projection opticals in the sides 22 and 23 are oblique in both the vertical direction and the horizontal direction to the screen.
FIG. 10 explains the relative positioning among projection lens 23a, CRT 23b and screen 60 in the oblique projection. The optical axis of lens has an angle of incidence .alpha. to the screen; consequently, CRT 23b needs to be tilted in accordance with the rules of optics by an angle .beta.. If the angle of incidence .alpha. changes, the tilt angle .beta. of the CRT should also be changed accordingly. If the projection distance is changed to vary the size of the projected image, the concentration angle .theta. as defined in FIG. 8 should also be changed. The concentration angle .theta. equals the angle of incidence .alpha. in FIG. 10. Therefore, the CRT tilt angle .beta. needs to be changed along with the change of the concentration angle .theta..
As already described above, a change in projection distance results in a change of the concentration angle of the projection lens, therefore the tilt angle .beta. of the CRT should also be changed. In the prior art projection lens units having an optical coupling as illustrated in FIG. 7, the space between the projection lens and the CRT 3 is watertight and assembled at a predetermined angle in the factory; it is impossible to change the CRT tilt angle after assembly. Therefore, various constituent components must be prepared in the factory in accordance with the prospective projection distances that are predetermined in several ranges. The CRT 3 is assembled with such components so as to have a specific tilt angle suitable to a range of prospective projection distances.
Disadvantageously, this means that the CRT needs to be manufactured in several models to cover the ranges of prospective projection distances. The increase in number of CRT models means that there is an increased number of components in the inventory. Further, production control and inventory control both for each CRT model and component can be very complicated inviting a substantial increase of expenses. In the event the CRTs of a certain distance model are in short supply, other CRT models in stock can not serve serve as substitutes if the distance range is different; then, the supplier has to manufacture the CRTs from the beginning, which takes a long time before delivery.
None of the configurations described above provides the important advantage of flexibly projecting pictures for a variety of distance ranges without manufacturing a number of different models. In particular, the prior configurations do not provide the advantages of a variable CRT tilt angle, a variable concentration angle and a minimal number of components.